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Full text of "Painting galvanized iron and other zinc surfaces / H.A. Nelson."

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The Painting of 
Galvanized Iron 



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Copyright, 1930 

The New Jersey Zinc Company 

New York, IN. Y. 



Painting 

Galvanized Iron 

and Other Zinc Surfaces 



H. A. NELSON 

Chief of Pigment Research Division 

The New Jersey Zinc Company 



The New Jersey Zinc Co. 

1 60 Front Street, New York 



Painting 

Galvanized Iron 

and Other Zinc Surfaces 



THE low cost of maintaining galvanized iron and sheet 
zinc structures is largely responsible for their choice 
for industrial building. In the case of galvanized iron, 
however, the zinc coating is penetrated by the weather in 
time; this time depending on the quality of the coating and 
the conditions of exposure. This exposes bare iron and makes 
it necessary to protect the building by the use of paint. After 
a galvanized surface has weathered, painting can ordinarily 
be carried out with complete assurance of good adherence 
if the proper quality of paint is used. On the other hand, 
it is often desirable to paint a newly erected galvanized iron 
or sheet zinc structure for decorative purposes or to follow 
out a uniform color scheme. Such cases require special con- 
sideration. 

factors governing 

adherence of paint to new 

galvanized iron and sheet zinc surfaces 

New galvanized iron and sheet zinc both present a very 
smooth surface which offers no pores or pits into which a paint 
can penetrate to obtain mechanical anchorage. Iron sheeting, 
on the other hand, usually has a rough surface. Further- 
more, it is not greatly expanded and contracted by tempera- 
ture variations. Changes in temperature cause expansion or 
contraction which is slight in the case of galvanized iron but 
more marked in the case of sheet zinc. This change has an 
effect on adherence of paint— especially so on a surface offer- 
ing no mechanical anchorage. As a result, an ordinary paint 
may peel in large sheets, leaving the bare metal exposed. 
Further difficulty may be encountered in painting unweathered 
zinc surfaces due to the presence of grease or dirt. 



Painting 

Galvanized Iron 

and Other Zinc Surfaces 



These difficulties can be overcome in a large measure by 
using the best type of metal priming paint for these surfaces. 

weathering galvanized iron 
and sheet zinc before painting 

Weathering for two to six months will ordinarily remove 
any grease present and at the same time roughen or etch the 
surface of the metal. This allows the paint to come in direct 
contact with and wet the metal and, also, provides a slightly 
roughened surface on which the paint can obtain some me- 
chanical anchorage. This weathering can be supplemented, 
often to good advantage, by cleaning and etching treatments, 
but such supplementary treatment as a rule need not be em- 
ployed. When it is desired to paint a zinc surface before it 
has weathered, it usually is necessary, before applying paint, 
to use a method of pretreatment in order to obtain a surface 
equivalent to one which has been weathered. 

pretreating galvanized iron and sheet 
zinc for painting, and recommendations 
for cases where pretreating is required 

Although widely recommended and used, pretreating 
methods have not been found wholly satisfactory because 
chemicals having an etching effect on the metal leave a de- 
posit of water soluble salts. This difficulty can be overcome 
by completely removing the salts from the pretreated sur 
face by a thorough brushing or washing with clear water. 
Some pretreating methods give entire satisfaction if properly 
carried out, and others are of little or no value. Naturally 



the most satisfactory pretreating method is one in which 
cleaning and etching take place in one operation. Suggested 
methods follow : 

(1) — Sand blasting is the most effective treatment for 
all conditions of the surface. This method cleans and etches 
in one operation. 

(2) — A more practicable and economical method of ob- 
taining a cleaned and etched surface in a single operation 
from a surface contaminated with grease is to apply liberally, 
with an oil free brush, an acidified mixture of denatured al 
cohol, toluol and carbon tetrachloride. This may be prepared 
approximately as follows: 

60 volumes denatured alcohol 
30 volumes toluol 
5 volumes carbon tetrachloride 

5 volumes commercial concentrated hydrochloric 
acid (muriatic acid) 
This treatment is especially effective and is highly recom- 
mended. 

(3) — Merely washing the surface with toluol, or some 
solvent in which the oils or greases are soluble, will serve in 
many cases, and especially for interior work. If the surface 
is not exposed to extreme temperature changes, a thoroughly 
cleaned zinc surface provides sufficient anchorage for the paint 
film, assuming, of course, that the paint is one that can be 
expected to adhere. 

(4) — If the surface has weathered or is originally quite 
free from oil or grease, brushing with, or dipping the surface 
in, a solution of copper acetate (6 ounces per gallon) in water 
etches the surface quite well. The weakness of this method 
is that it does not dissolve oil or grease and hence will not 
etch greasy surfaces thoroughly. 



Painting 

Galvanized Iron 

and Other Zinc Surfaces 



Whenever any pretreating method is used the surfaces 
should be completely dry and free from dirt, grease and sol- 
uble salts before the application of paint. Natural weathering 
over a period of two to six months has obvious advantages 
over any of the above treatments. They supplement natural 
weathering, however, in a very satisfactory manner. 

special pretreated galvanized sheets 

Galvanized sheet which has been annealed to give a matte 




Figure 1 
Galvanized Iron Structure Painted With Two Coats Zinc 
Dust-Zinc Oxide Paint 



surface is now on the market. As to paintability it approxi- 
mates a naturally weathered zinc surface. 

paints for 

galvanized iron and sheet zinc 

Extensive tests carried out over a number of years indicate 
that those paints classed as "Metal Primers" are, as a whole, 
better adapted for priming zinc surfaces than other general 
classes of paints. However, a paint for this particular type 
of service need not be of the rust inhibitor class, as for iron 
and steel surfaces. The suitability of a paint for zinc sur- 
faces depends largely on its physical properties, while on 
ordinary iron or steel surface a successful paint must have 
certain chemical as well as physical properties to prevent 
corrosion. The outstanding requirements of a paint for zinc 
surfaces are: (1) that it be highly distensible and retain this 
property over a long period of exposure to weathering, and 
(2) that it have the property of thoroughly wetting the sur- 
face at least at the outset (whether after the film has oxidized 
the adherence is greatly influenced by factors other than me- 
chanical gripping of the surface, we are not prepared to say). 

scope of tests 

During the past few years a large number of tests have been 
carried out on paints applied to pretreated and untreated sheet 
zinc and galvanized iron panels, exposed outdoors at a 45 
angle facing south. Other tests were carried out on a number 
of buildings sided with these metals, and on test panels ex- 
posed to accelerated weathering. The results of the various 
methods of testing are in relatively close agreement and may 
be considered conclusive in as far as they go. 



Painting 

Galvanized Iron 

and Other Zinc Surfaces 



Figure No. 1 illustrates a galvanized structure painted 
with two coats of zinc dust-zinc oxide paint. This paint was 
applied by spray gun five years ago to a weathered galvanized 
surface. The paint is exposed to particularly severe conditions, 
the building being located in the smoky and gaseous atmos- 
phere of a large industrial plant. A close inspection of this 
building revealed absolutely no cracking, scaling or peeling of 
the paint after five years' exposure. 

A section of weathered galvanized iron fence around an 
industrial plant is illustrated in Figure No. 2. This fence 
was sprayed with one coat of zinc dust-zinc oxide paint four 
years ago. Figure No. 3 illustrates a close-up view of a rep 
resentative section of this fence and clearly shows the excel 
lent condition of the paint after four years' outdoor exposure. 

The following key is for the treatments used on th< 
panels shown in Figures No. 4 to 7 inclusive: 
"A" cleaned with benzol but unetched. 

treated with copper acetate solution (6 02 pel 
"E" etched with the following mixture: 
35 parts (by volume) toluol 
65 parts denatured alcohol 
5 parts hydrochloric acid 
(In subsequent tests the addition of 5 parts carbon ' 
chloride was found to speed up the action of I 
Hence it is included in the formulae recommendi ■- e 7.) 

cleaned with a solution consisting of: 
250 cc concentrated sodium carbonate solution 
500 cc 3'' sodium silicate solution 
G weathered for three mor • ng 

H Light sand I 
In each case the solution- 
paint brush. After the panel 







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Figure 2 
View of Zinc Dust Painted Galvanized Fence Around Industrial Planl 



Figure 3 

Close-up of a Small Representative Area of Fence 
Illustrated in Figure No. 2 



Painting 

Galvanized Iron 

and Other Zinc Surfaces 



were rinsed off with water, following which the panels were 
again dried carefully before any paint was applied. 

Figure No. 4 illustrates a series of galvanized iron test 
panels exposed for over two years at an angle of 45 facing 
south. The panels were pretreated as noted above, and then 
primed with one coat of red lead and second and third coated 
with two white finish paints. (Zinc Oxide-Lithopone base 
paint on left half of each panel and white lead on right half 
of each panel.) It is at once apparent from this illustration 
that only on the sand-blasted panel *'H" is the paint showing 
any satisfactory degree of adherence. This test indicates that 
red lead is not a satisfactory primer for untreated or etched 
galvanized surfaces, with the exception of those that are sand 
blasted. 

The panels in Figures 4 and 5 were all exposed for the 
same length of time, and in the same manner. The panels in 
Figure 5 were untreated and pretreated by the various methods 
described and painted with an iron oxide zinc oxide paint, 
and then finished on each half with two coats of two white 
finish paints as on the panels in Figure No. 4. This prin i 
considered very good for galvanized iron, and in the past, has 
given quite effective service in the plants of the New J< 
Zinc Co. Panels "A,' "E," "G." and "H" are reasonably 
from peeling, *'H" being by far the outstanding panel of the 
set. Here, with the exception of "E" the chemically etched 
panels show most evidence of failure. These panels are con- 
siderably superior to those appearing in Fig. 4. 

Figure No. 6 illustrates a series of treated galvanizefi 
panels primed with a zinc dust-zinc oxide 
in the bulletin "Metallic Zinc Powder in I int." 



Representative gal- 
vanized iron panels 
oi a test series after 
Z 1 /* years' exposure 
at a 45 angle facing 
south 

The primer is a 
commercial pure red 
lead paint. 

Left Half— Finished 
with two coats of a 
Zinc Oxide - Litho- 
pone Paint. 

Right Half — Fin- 
ished with two 
coats of white lead 
paint 



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Figure A 




13 



Painting 

Galvanized Iron 

and Other Zinc Surfaces 



I 



y 



Representative gal- 
vanized iron panels of a test 
series after 2% years' ex- 
posure at a 45 : angle facing 
south. 

Primed with an Iron Oxide- 
Zinc Oxide Paint. 

Left Hand — Finished with 
two coats of a Zinc Oxide- 
Lithopone Paint. 

Right Half — Finished with 
two coats of white lead 
paint. 



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Figure 5 




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Representative gal- 
vanized iron panels of a test 
series after 2 1 /^ years' ex- 
posure at a 45° angle facing 
south. 

Primed with a Zinc Dust- 
Zinc Oxide paint. 


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Left Half— Finished with 
two coats of a Zinc-Oxide- 
Lithopone Paint. 










Right Half— Finished with 
two coats of white lead 
paint. 






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Figure 6 


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Painting 

Galvanized Iron 

and Other Zinc Surfaces 



The finishing coats are the same as used on the panels in the 
preceding illustrations. These panels are pretreated in the 
same manner as the other series of panels and were also ex 
posed for over two years to weathering at an angle of 45 
facing south. It is quite apparent that the condition o! 
paints on these panels is by far superior to the oth< 
on the panels in the preceding photographs. Compared with 
the other paints, panels "A." "E f " "G." and "H" in this set are 
in excellent condition. These panels are outstanding, with the 
sand blasted and weathered panels generally in better condi- 
tion than the chemically etched sur- I 
Throughout our experience zinc dust-zinc oxide paim 
shown unusual adhering properties to new galvanized iron 
that had no treatment or weathering whatsoever prior to paint- 
ing. Zinc dust zinc-oxide paint adhere uch sur- 
faces than many other primers do over treated surfa. i 

The galvanized iron panels illustrated in I \'o. 7 

were treated and painted throughout with three co.r 
zinc dust zinc oxide paint referred to above. Tl 
ity over the rest is at once apparen' iutely no 

• nee of peeling on any panel. (The I 
photograph are caused by high Ligl nade 

with a knife, indicate that the adherent* 
face "A" is equally a the adherence or 

chemically etched Of the chemical 

Thi 
zinc dust-zinc oxide prin « 
methods becomes lean 

and dry. Mechanical roughening of 




Representative gal- 
vanized iron panels of a test 
series after 2 I < 4 years' ex- 
posure at a 45 angle facing 
south. 

Three coats of gray Zinc 
Dust-Zinc Oxide Paint. 
Paints formulated for use as 
primer, second and third- 
coat paints. 



Figure 7 




Painting 

Galvanized Iron 

and Other Zinc Surfaces 



ing will, of course, produce results surperior to those obtained 
on either weathered or chemically etched surfaces. 

Of the etching solutions, the acidulated toluol-alcohol 
mixture, used on panels "E," seems to be in a class by itself. 
As previously mentioned, this mixture is designed to clean 
and etch in one operation. By slightly modif\m L this with the 
addition of a small amount of carbon tetrachloride (a- 
scribed on page 7) greater etching efficiency is developed and 
even better results are obtained. 

Similar results were obtained in tests run on zinc 
panels. The use of a priming paint adapted to tin 
eliminates most of the troubles. There should be no h< 
tion in painting zinc surfao i th< « tc mentioned above 

demonstrate whai ury results can be obtain. 

a priming paint is used which i h ned for the purpo i 

Commercial application at the plants of Tl - 
Zinc Company and others corroborate • • i 
In painting ordinary black iron for permanerr 
tain precautions must be takei 

and free from grease, dirt and loose scale. Also, paint 
be selected that adht ing and durable. In 

painting zinc i «ntion can be given t< 

factors governing good painting practice. In ad<: 
should be chosen that has the added property of ftdl I 
a zir. e — and it is quit* at a paint tf 

gooc n black iron does not \y have the q 

fications I for a paint for galvar 

lie Zinc Powder (Zir. 

Zinc Oxide, has \ ;on for many ye. 

the qualificat Mai for I 



galvanized and sheet zinc surfaces— and incidentally black 



iron. 



lacquering and 
enameling sheet, cast zinc, 
and galvanized iron surfaces 

The use of quick drying enamels and lacquers for coating 
zinc surfaces presents a somewhat more difficult problem. 
Ordinarily they cannot be depended upon to adhere well to 
such metallic surfaces when these are untreated. In any case, 
far more satisfactory results are obtained if zinc surfaces are 
coated with oil base primers. The best procedure in such 
cases is to bake the priming coat on the metal. 

Before the application of any coating to the surface, the 
metal should be thoroughly cleaned. The most effective 
method is to immerse the article in a cleaning solution of the 
following composition: 

200 cc Water 
30 grams tri-sodium phosphate 
4 grams sodium hydroxide 
This solution should be used at a temperature of 70 to 
80°C and be stirred with a motor agitator. After rinsing (hot 
water) the surface may be etched by immersing from I 4 to ] 
minutes (one minute is suggested in the case of sheet zinc or 
zinc articles) in a solution of the following composition : 

1,000 cc water 
200 cc hydrochloric acid (commercial muriatic acid) 
20 grams ammonium nitrate 
Rinse with water and dry thoroughly before coating. It 
is important to remove all water soluble salts. 



19 



Painting 

Galvanized Iron 

and Other Zinc Surfaces 



Good results can also be obtained by brushing on the 
surface the denatured alcohol mixture described under (2) 
Page 7. The very best results are produced by roughening 
the surface by some mechanical means such as a light sand 
blast. This treatment is suggested whenever possible. 

conclusions 

Some question remains as to the best method of treating 
zinc surfaces — both galvanized and sheet zinc — before paint- 
ing. However, many tests and observations seem to prove 
that: 

1. Zinc surfaces need pretreatment when most priming 
paints are used. 

2. Certain methods of pretreatment, particularly sand- 
blasting and natural weathering, give best results with any 
paint. 

3. Chemical etching is less effective, but has the advantage 
of being quicker than weathering and less costly than sand- 
blasting. The denatured alcohol mixture described is recom- 
mended for use. 

4. There are priming paints available which give satis- 
factory results on treated or untreated zinc surfaces, assuming 
that the latter are clean and dry. Experience and close obser- 
vation have shown that zinc dust-zinc oxide paints are em- 
inently efficient. 

5. There need be no more hesitation in painting zinc 
surfaces than in painting any ether metallic surface, if prim- 
ing paints are properly selected and the usual precautions are 
taken that apply in the painting of any metal. 



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